Tissue products, such as facial tissues, paper towels, bath tissues, napkins, and other similar products, are designed to include several important properties. For example, the products should have good bulk, a soft feel, and should have good strength. Unfortunately, however, when steps are taken to increase one property of the product, other characteristics of the product are often adversely affected.
To achieve the optimum product properties, tissue products are typically formed, at least in part, from pulps containing wood fibers and often a blend of hardwood and softwood fibers to achieve the desire properties. Typically when attempting to optimize softness, as is often the case with tissue products, the papermaker will select the fiber furnish based in part on the coarseness of wood fibers. Pulps having fibers with low coarseness are desirable because tissue paper made from fibers having a low coarseness can be made softer than similar tissue paper made from fibers having a high coarseness.
Fiber coarseness generally increases as fiber length and fiber surface area increase. Thus, the softness of tissue products can be improved by forming the tissue products from pulps comprising primarily short fibers, as they typically have a lower coarseness relative to long fibers. Unfortunately, tissue paper strength generally decreases as the average fiber length is reduced. Therefore, simply reducing the pulp average fiber length can result in an undesirable trade-off between product softness and product strength.
Tissue products having improved softness can also be formed from pulps comprising fibers from selected species of hardwood trees. Hardwood fibers are generally less coarse than softwood fibers. For example, those skilled in the art recognize that bleached kraft pulps made from eucalyptus contain fibers of relatively low coarseness and can be used to improve the perceived softness of tissue products. Unfortunately, because kraft pulps made from a single species such as eucalyptus are preferred by papermakers attempting to make soft, durable tissue products, they are in high demand and therefore more expensive than certain pulps which tend to comprise fibers generally having inferior coarseness properties. Examples include pulps which are derived by mechanical pulping regardless of the source species and recycled pulps which invariably contain a mixture of fiber types and species. Such blends are particularly prone to having relatively high coarseness compared to their average fiber length.
The papermaker who is able to obtain pulps having a desirable combination of fiber length and coarseness from fiber blends generally regarded as inferior with respect to average coarseness and uniformity of fiber properties may reap significant cost savings and/or product improvements. For example, the papermaker may wish to make a tissue paper of superior strength without incurring the usual degradation in softness which accompanies higher strength. Alternatively, the papermaker may wish a higher degree of paper surface bonding to reduce the release of free fibers without suffering the usual decrease in softness which accompanies greater bonding of surface fibers. As such, a need currently exists for a tissue product formed from a fiber that will improve softness without negatively affecting other important product properties, such as strength.